Trying not to worry

The vet called today, and they now think that Random might have heartworms?!? Heartworms are spread by the bite of an infected mosquito, so I'm wondering how he could have gotten infected indoors. Because heartworms are incurable and fatal in cats, I had been giving him preventative heartworm meds for a while, but decided that I was being too paranoid. Unfortunately, the vet woke me up, so I didn't have the wit to mention that he's an indoor cat. He isn't exhibiting the normal heartworm symptoms.

I okayed some more tests. If they say definitively that he has heartworms, I want another set of tests done, a second opinion, or both.

I'm also wondering if they could have mixed Random up with another cat. Unfortunately, I don't remember the vet's name....